2. Learning Objectives
Study the evidence supporting the
continental drift theory
Discuss how the ideas of continental drift
and seafloor spreading
Explain how plate movements are affected
by volcanic activities and earthquakes
3. Continental Drift
Proposed by Alfred Wegener in 1912
Stated that the continents we know today
had once been joined together in a single
landmass (Pangaea)
People during his time thought that this
idea was radical too radical so Wegener
came up with a list of evidences to support
his contention
4. Continental Drift: Evidences
The fits of the continents
The similarity of fossils in different
continents
The similarity of rock type and age along
matching coastlines
The continuity of geologic features from
continent to continent
The presence of coal seams in Antarctica
Tillites
5. Continental Drift: Rejection
The mechanism presented of Wegener
wasn’t found plausible.
The mechanism centered on using the
gravitational pull of the moon as the main
reason that caused the motion the
continents.
The explanation provided was not
accepted.
What could be wrong?
6. Before we proceed to talking about
SEAFLOOR SPREADING, we first
talk about PALEOMAGNETISM…
7. Paleomagnetism
Some rocks contain magnetic minerals
known are magnetite.
The mineral is made of the elements iron
and oxygen.
The magnetite found in the rocks are
capable of recording the magnetic field of
earth.
Such record proved that continents did
move somewhere in the past… But how?
8. Deep Discoveries (1950 – 1960)
Presence of a belt of underwater
mountains (oceanic ridges) that encircles
the globe
Presence of a central valley (rift valley) at
the summit of the oceanic ridges
Oceanic ridges (composed of volcanic
rocks) give off an abnormally high amount
of heat
Earthquakes in the deep sea areas were
found to be associated with trenches
The seafloor is just 170 million yrs old
9. All of the discoveries listed in the
previous slide were duly accounted by a
theory proposed by Harry Hess… the
SEAFLOOR
SPREADING
12. The Theory
Holds that the outermost rigid layer of the
earth is broken into several segments
called plates, and these plates are in
motion.
The movement of the plates are described
by the three types of plate boundaries;
Divergent plate boundary
Convergent plate boundary
Transform boundary
13. Divergent Boundary
Place where two plates are moving apart
from each other just like the boundary
found in the eastern part of Africa
Creates huge cracks or valleys that would
later be filled up with water from rain or
nearby streams (narrow and elongated
lakes are created)
Explains the formation of Lake Malawi,
Lake Tanganyika and the Red Sea
14. Transform Boundary
Two neighboring plates are neither
spreading apart nor colliding with each
other
The two interacting plates slide past each
other i.e. San Andreas Fault
15.
16. Convergent Boundary
Place where two plates move toward each
other
Three possible cases could happen long
these boundaries;
Two continental plates may collide
Two oceanic plates me collide
A continental plate and an oceanic plate may
collide
17. Transform Boundary
Two neighboring plates are neither
spreading apart nor colliding with each
other
The two interacting plates slide past each
other i.e. San Andreas Fault
But what is causing all these movements?
20. Fracture
Common on the uppermost part of the
crust
Promoted by conditions of low pressure
and temperature
Comes in two types – faults and joints
21. Faults
Breaks along which there is considerable
movement
Are of two types:
Dip-slip (involves the movement of blocks of rock
mainly in the vertical direction)
Strike-slip (movement is in horizontal direction)
22. Folding
Bending of rocks
Happens deep within the crust
Rocks deep within become thicker as they
are squeezed, or become thinner when
pulled apart
Promoted by high pressures and
temperatures
